By Cassie Shortsleeve

Period pain is real – and (as you’ve probably noticed) not created equal.

We all have one friend who hardly notices when Aunt Flo’s in town, and another one who calls into work sick because of bad cramps. What gives?

Menstrual cycles vary – in length and intensity, says Alyssa Dweck, an assistant clinical professor of obstetrics, gynaecology and reproductive science at the US Mount Sinai School of Medicine, and author of V Is for Vagina. “We have a couple of benchmarks that we often give people,” she says. The typical cycle, she explains, is between 21 and 35 days, and the typical period is between two and seven.

Some ladies have horrible periods because of a medical condition called endometriosis, says Dweck. It happens when tissue that normally lines the uterus grows outside of it – and there are about 176 million cases of it worldwide. But otherwise healthy women can have worse-than-normal cycles, too.

Here, six reasons your period could be lighter – or heavier – than your girlfriends’.

1. Your Diet Stinks

So you fell off the wagon for a few months – it happens. But a lousy diet doesn’t just affect the scale. “If women have a few months where their diet is really bad, that can alter menstrual flow,” says Dweck. One study of school-aged girls found that students who ate more junk food also suffered from more premenstrual symptoms. Other research demonstrates that good-for-you nutrients – like omega-3s and calcium – reduce period pains.

2. Your Age Is To Blame

“For the first years after menstruation begins, there is no ovulation and longer cycles are common,” says Mamta Mamik, an assistant professor of obstetrics, gynaecology and reproductive science at the Icahn School of Medicine at Mount Sinai. As we age, periods normally become more regular and shorter, she adds. That is until your 40s or 50s. Some women experience uncomfortable and haphazard periods as they enter perimenopause – the stage right before menopause, says Dweck, who adds that this could include worsening pain or bad bleeding.

Read More: 7 Facts That Could Change Your Mind About Menstrual Cups

3. You Don’t Exercise – Or You Exercise Too Much

There’s a fine line when it comes to exercise and your period. “Women who do not exercise at all may have worse periods than women who do exercise,” says Dweck. The good news is that the fix may be as simple as little movement. Ladies with bad cramps who exercise usually ease their cramping – cardio can improve your flow and probably lighten your period, she says.

Just don’t take it too far: “Excessive exercise can lead to absent or reduced periods due to the effect on the hypothalamus, which can suppress periods,” says Mamik. This, she adds, has to do with a low body mass index (BMI) – which can lead to amenorrhea (total lack of a period).

4. Weight Woes Are At Play

This one’s hard to tease out, but Dweck says that – in general – overweight women tend to have worse, more irregular periods. Research suggests that this could be due to hormonal factors, such as amounts of insulin and oestrogen.

The key to smooth sailing is landing somewhere in the middle. “We’ll see women with eating disorders or who are athletes with BMIs below 18 or 19, and they will lose their periods,” says Dweck. Some theorise this is nature’s way of preventing pregnancy during times of stress, she says. Long term, not ovulating exposes you to less oestrogen – which can have a negative effect on the bones, putting you at risk for osteoporosis.

Read More: What It’s Like To Wear A Menstrual Cup

5. You’re Not On The Pill

Docs don’t just prescribe the Pill to prevent pregnancies: “Taking oral contraceptives makes periods more manageable and not as heavy,” says Mamik. It’s not just the Pill either – while some people report cramping after IUD insertion, studies suggest that the devices don’t increase cramping, and contrary to that, many report a decrease in the uncomfortable stomach pains.

6. Something More Serious Is Going On

None of these explanations fit the bill? “Periods may be affected by various underlying pathological factors in the uterus,” says Mamik. Fibroids (abnormal growths in or on the uterus) and polyps (benign outgrowths from the lining of the uterus) can both make periods heavier and more painful than normal, she says.

Metabolic syndromes like polycystic ovarian syndrome can also cause irregular periods and prolonged, excessive bleeding, says Mamik. The bottom line: if you’re getting your period more often than every three weeks or less often than every five weeks, bleeding in between periods, or going through multiple tampons an hour, it’s time to make an appointment with your gynae, says Dweck.

Looking fore more info on your period? Here’s everything you ever wanted to know about your menstrual cycle, plus seven legit reasons why your period could be late… beside pregnancy.